Just noticed this yesterday when i updated my Laptop via Windows Update, that this update updates the explorer.exe, which will effect if you got a Custom start button installed.
After installing the update, you'll have to restart and when you are back from the restart your start button will now be changed to the default Windows 7 Start button, so you'll have to change it again if you wanna have a custom start button.
Note: I dont know if this update is out for x86 based systems
Updated for Windows 7 for x64-based systems (KB977074)
Just wanted to let you all know